The girls kept coming. All afternoon, in a warm, air-conditionless loft in SoHo, lithe young women arrived, with chiseled features and impossibly long legs, wearing minis and heels, one after the other. The elevator door opened directly into the loft space, depositing groups of girls, as everyone in the room kept calling them, whisking others away.

The enviably difficult job of choosing about 15 models from the close to 100 who showed up last Thursday afternoon fell to the two most unassuming people in the room. Jeffrey Costello, 42, and Robert Tagliapietra, 30, the duo behind the eponymous Costello Tagliapietra label, were busily, nervously preparing for their debut Fashion Week show, which will take place tomorrow, the first day of Fashion Week. Picking the models to represent their line on the runway was just one of the myriad details they had to attend to.

Clad in jeans and sneakers, Messrs. Costello and Tagliapietra are both sweet-tempered, burly, bearded men who live and work out of a combined apartment and work space in Carroll Gardens, Brooklyn. They look more like they might have been called in to move furniture than like the creative brains behind a line of swingy, sexy dresses that Vogue recently dubbed a sartorial essential.

Auditioning models is a quick business. At last week’s Costello Tagliapietra casting, a young woman would come in and hand her modeling book to the casting director, Natalie Joos, who took a Polaroid of her against the white wall background. If Ms. Joos and Messrs. Costello and Tagliapietra liked what they saw, the girl was asked to take a trial runway walk down the loft’s hall, past a kitchen and a bedroom door. Then she was thanked and dismissed. All this generally took less than three minutes.

Messrs. Costello and Tagliapietra wore worried expressions on their faces as they studied the models. “This is so hard. You feel badly when you ask one to walk and another is just standing there,” Mr. Tagliapietra said. “They’re all beautiful, but some have the look we’re going for, and some don’t.”

The look they’re trying to capture is “ethereal, romantic-looking, not so hard-edged,” said Mr. Tagliapietra, in keeping with the spirit of their drapey, below-the-knee designs. Composed entirely of jersey, a fabric to which Messrs. Costello and Tagliapietra are devoted (“it gives the clothes a certain ease,” said Mr. Costello), their creations somehow manage to marry the structured elegance of a vintage look with a looser, modern sensuality.

“We like them to have an ageless, almost pulled-out-of-your-grandmother’s-closet look,” said Mr. Costello. “Even if it reminds you of an older decade, we try to give it a modern twist,” Mr. Tagliapietra noted, pointing to one dress’s exaggerated stitching, low neckline, and back slit.

The reference to grandmothers comes quickly to both men, who learned to sew under the tutelage of their family matriarchs. (In a strange coincidence that Messrs. Costello and Tagliapietra didn’t learn about until three years ago, both of their grandmothers worked as seamstresses for the designer Norman Norell in the 1960s.) These women instilled in them time-honored tailoring techniques, which distinguish the Costello Tagliapietra label today: bias strips of organza inserted at the seam, underarm gussets, and pressed-open seams.

“I still have her sewing machine,” Mr. Costello said of his late grandmother. “Each season, I create one piece on it.”

Messrs. Costello and Tagliapietra met in 1994, and began working together almost immediately. At the time, Mr. Tagliapietra had just finished a degree in painting at Parson’s School of Design. Mr. Costello was working as a freelance designer for clients such as Madonna, Patty Scialfa, and the actress Debi Mazar. (One of his biggest breaks was outfitting Ms. Mazar for her role in “Goodfellas.”) In the past decade, the duo has worked on a variety of projects together, from costume design for film, music videos, and theater productions to a lingerie line, continuing to create made-to-order designs for private clients on the side. “Two years ago, we just decided: ‘Let’s do a full collection,'” Mr. Costello said.

And while the designs are presumably the stars of the show, at the casting last week a handful of autumnal-hued dresses (“we think of them as memories of color, rather than the color itself,” explained Mr. Tagliapietra) took a decidedly backstage role. Several hung on a rack shunted off to the side, as Messrs. Costello and Tagliapietra and Ms. Joos studied the models, offering suggestions (“try a softer walk,” they urged again and again), discussing the possibilities. “She’s beautiful, but…” Mr. Costello said, trailing off. “Too European,” Ms. Joos said.

At one point, more than a half-dozen women quietly awaited their turn on couches by the loft’s sun-drenched windows (some exchanging their Converse high tops or Chinese slippers for a pair of heels). It was difficult not to feel as if one had stumbled upon another planet, solely inhabited by gorgeous, 6-foot-tall, willowy women.

Soon, Kelly Cutrone, the head of People’s Revolution, the public relations firm orchestrating the Costello Tagliapietra show, bounded into the loft, which also happens to be living quarters for her and her 2-year-old daughter. (That explained the doll’s house and cradle, pushed up against the far wall.) Ms. Cutrone was wearing a sleeveless T-shirt that read “No More Fashion” on one side, and “The Fashion Industry Sucks” on the other. “I’m selling these for $200 a pop,” she said, laughing.

During a lull in model watching, Ms. Cutrone pulled Messrs. Costello and Tagliapietra aside to show off her latest sartorial purchases for her daughter, including a pint-sized Sonia Rykiel suit. When the two designers returned, they acknowledged their nerves about the fashion show, which is expected to draw several hundred people, and their attendant hopes for the line’s future. At present, their designs are sold to private clients from their Carroll Gardens studio and carried by Barneys Japan (they retail from about $500 to $2,500). They would be thrilled to garner more retail exposure, as well as to secure financial backing for the company.

One model, Cintia Dicker, 17, from Brazil, an ethereal beauty with long cinnamon-colored hair, caught Messrs. Costello and Tagliapietra’s eyes. They asked her to try on a dress, and Ms. Dicker, wearing a plain T-shirt and a miniskirt, disappeared. Several minutes later, she reemerged, transformed, in a sumptuous rust colored dress, one of the frocks that had been hanging forgotten on a rack in the corner. The dress had a beautifully loose, draping look, with a deep V in front and a slit in the back. Ms. Dicker was asked to stand by the loft’s huge windows overlooking Grand Street, and in the afternoon light, she glowed.

Someone complimented Mr. Costello on the design, and a sheepish grin lit up his face. He seemed more relaxed than he had been all afternoon. “It just nice when everything comes together, and it looks the way you hoped it would,” he said quietly.